This invention relates to non-glass ion selective electrode systems used in the measurement of pH.
Electrochemical means for measuring the pH levels of liquid systems are well known. Glass sensors having membrane type of electrodes are commonly used as standards for pH measurements because sensing instruments with glass sensors are widely available commercially and the measurements from the glass sensors are predictable and provide good measuring capability. Glass sensors are fabricated as three-dimensional articles, with many serviceable parts. While demonstrating good measuring capability, these three-dimensional articles are more expensive and complex to manufacture as well as operate as compared to planar format sensors.
Non-glass based pH sensors have also been described in the literature. The non-glass sensors are typically prepared using solvent polymeric membranes (such as those described in Analytical Chemistry, 1986, 58, 2285-2289, hereby incorporated by reference). Within the category of non-glass sensors are those of a planar format which is typically smaller than glass sensors and much less expensive to manufacture as well as operate. Examples of planar sensors can be found in U.S. Pat. Nos. 5,554,272; 5,702,575; and 5,786,830, each of which is hereby incorporated by reference. Instruments containing planar sensors are available commercially. The planar format of the sensors typically comprises relatively thin layers of materials which are applied to substrate bases using thick-film or thin-film techniques, including, for example, silk-screen printing.
Non-glass sensors for the measurement of pH offer an economical alternative to glass sensors, but the membrane-based pH sensors have been associated with a scatter of results commonly referred to as pH bias. As used herein, pH bias is defined as equal to measured pH using a membrane based sensor minus measured pH using a known standard which is typically a measurement obtained from a glass based pH sensor. If the pH bias is too great, the error has a potential to approach clinical decision levels. Accordingly, a method of decreasing the pH bias associated with membrane based sensors is needed.
The present invention provides a means to substantially decrease the pH bias that is commonly observed with pH data obtained using instruments that have a solvent polymeric membrane based pH electrode. The pH bias is reduced by utilizing the discovery that the presence of protein in a liquid sample induces an apparent interference with the pH value. According to the invention, provided is a method of measuring pH in an ion selective electrode based sensing instrument that employs a polymeric membrane based pH electrode, said method comprising measuring the pH of a protein containing liquid sample using said pH electrode to obtain a raw pH measurement and correcting said raw pH measurement for interference induced by the presence of protein in said liquid sample.
Also provided is a method of reducing the pH bias demonstrated by a planar polymeric membrane based pH ion selective electrode, said method comprising (a) measuring the pH of a protein containing liquid sample using said pH electrode to obtain a raw pH value, (b) obtaining a bicarbonate value of said sample either directly or indirectly, (c) obtaining a corrected pH value by inserting the raw pH value and bicarbonate value into an Equation [1*]
{Corrected pH=Raw pHxe2x88x92(a*[HCO3xe2x88x92]2+b*[HCO3xe2x88x92]xe2x88x92c)}xe2x80x83xe2x80x83[1*],
wherein a*, b* and c are empirically derived coefficients and [HCO3xe2x88x92] represents said bicarbonate value.
Also provided is a method of measuring pH using an ion selective polymeric membrane based electrode, said method comprising (a) contacting a protein containing liquid sample with a polymeric membrane based ion selective electrode specific to pH; (b) contacting said liquid sample with an electrode to derive a bicarbonate value; (c) contacting said liquid sample with at least one reference electrode either directly or indirectly; (d) connecting said exposed contact area of sensor with a sensing instrument; (e) providing an electrical current from said sensing instrument through said reference electrode to derive a raw pH value; (f) correcting said raw pH value by utilizing in said sensing instrument the Equation [1*]
Corrected pH=Raw pHxe2x88x92(a*[HCO3xe2x88x92]2+b*[HCO3xe2x88x92]xe2x88x92c)xe2x80x83xe2x80x83[1*]
wherein a*, b* and c are empirically derived coefficients and [HCO3xe2x88x92] represents said bicarbonate value.
Also provided is a method of correcting for the presence of protein in liquid samples when using a system comprising a planar polymeric membrane pH ISE electrode, said method comprising using a [HCO3xe2x88x92] value measured from said liquid sample in a sensing instrument by inserting a correcting equation into the software of said instrument wherein said equation utilizes the raw pH obtained from said pH ISE electrode and the [HCO3xe2x88x92] value. Preferably the pH bias (as verified using a pool of 200 samples run on said instrument and using a three dimensional glass sensor as the standard) is reduced to a level of less than xc2x10.04 pH units, more preferably equal to or less than xc2x10.02 pH units.
The invention provides an improvement for any system utilizing solvent polymeric membrane based ion selective electrodes where pH is included as one of the analytes of interest. By incorporating a correction equation into the ion selective electrode based system, pH bias may be substantially lessened. The correction equation may be easily incorporated into the software used in electrode systems that include a membrane based pH electrode by techniques well known to those skilled in the art.